The present invention relates to a valve characteristic estimation device for an internal combustion engine provided with a valve characteristic adjustment mechanism for varying valve characteristics including at least one of the valve open period and lift amount of an intake valve. The present invention further relates to a controller for an internal combustion engine provided with such a valve characteristic adjustment mechanism that controls the internal combustion engine based on the detected value from a sensor for detecting the current status of the value of the valve characteristic.
A valve characteristic adjustment mechanism is a well-known mechanism incorporated in an internal combustion engine. The valve characteristic adjustment mechanism variably adjusts a valve characteristic such as the valve open period and lift amount of an intake valve according to the operation state of the engine.
In an internal combustion engine provided with such a valve characteristic adjustment mechanism, the valve open period or lift amount of the intake valve is adjusted to change the condition of the air that flows into the corresponding cylinder. Such an internal combustion engine includes a sensor for detecting the operating status of the valve characteristic adjustment mechanism. More specifically, the sensor detects the current valve open period and lift amount of the intake valve. The internal combustion engine sets a target opening degree for the throttle valve (target throttle opening degree) based on the detection results of the sensor. The amount of overlap of the intake and exhaust valves also changes when the valve open period or the lift amount of the intake valve is changed. Thus, the internal combustion engine, which includes the valve timing adjustment mechanism for variously adjusting the valve timing of the intake valve, also sets a target value for the valve timing (target valve timing) based on the detection results of the valve open period and the lift amount of the intake valve. Further, the valve characteristic adjustment mechanism, which adjusts the valve open period and the lift amount of the intake valve, is feedback-controlled so that the actual valve characteristic detected by the sensor coincides with the target valve characteristic that is set in accordance with the operation state of the engine.
If, for example, an abnormality occurs in the sensor and direct detection of the valve open period and the lift amount of the intake valve are disabled, the engine control based on the detected values of the valve open period and the lift amount of the intake valve may not be executed properly. More specifically, the target throttle opening degree or the target valve timing may not be set correctly, or the feedback control of the valve characteristic adjustment mechanism may not be executed correctly. As a result, the engine output may not follow the driver's accelerator operation and the amount of intake air may become insufficient. In some cases, this may result in a misfire and make it difficult to drive the vehicle to a safe place. To avoid such a situation, it is preferable that the internal combustion engine have a means for determining the valve open period and lift amount of the intake valve even when direct detection of the valve open period and lift amount of the intake valve with the sensor is disabled.
Conventional methods for estimating the valve characteristic, such as the valve open period and the lift amount of the intake valve, without relying on direct detection with a sensor are described, for example, in Japanese Laid-Open Patent Publication Nos. 2000-282901 and 2000-314329. The methods described in these publications use a predetermined table associated with the engine speed, the intake air amount, and the valve characteristic. By referring to the table, an estimated value of the valve characteristic of the intake valve is calculated based on the detected intake air amount and engine speed. By estimating the valve characteristic indirectly from the detected engine speed and intake air amount, engine control may be continuously executed based on the roughly determined valve characteristic of the intake valve even if direct detection of the valve characteristic with the sensor is disabled. This enables the vehicle to be driven to a safe place.
However, recent internal combustion engines for vehicles execute more sophisticated intake air control. When employing the conventional methods described above in an internal combustion engine that executes such sophisticated intake air control, the valve characteristic of the intake valve cannot be accurately estimated.
For example, in an internal combustion engine provided with the above valve characteristic adjustment mechanism, the intake air amount may be adjusted not only by adjusting the throttle opening degree but also by adjusting the valve characteristic of the intake valve. Thus, the same intake air amount may be realized by various combinations of the throttle opening degree and the valve characteristic. From such various combinations, the optimum combination of the throttle opening degree and the valve characteristic may be selected in accordance with the engine operation state to obtain a desired intake air amount. This enables the execution of a more sophisticated intake air control. Under such sophisticated intake air control, the throttle opening degree is set differently depending on the operation state of the engine even when the engine load and engine speed are constant. This changes the condition of the intake air in accordance with the set throttle opening degree. Further, if the internal combustion engine is provided with a valve timing adjustment mechanism for variably adjusting the valve timing of the intake valve in accordance with the engine operation state, the condition of the intake air changes in accordance with the operating status of the valve timing adjustment mechanism. In this way, when there are a large number of factors that change the condition of the intake air, the valve characteristic, such as the valve open period and the lift amount of the intake valve, may not necessarily be directly determined from the engine speed and the intake air amount. Thus, the above conventional methods cannot accurately estimate the valve characteristic of the intake valve.